In an image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes an optical beam scanning apparatus that forms an electrostatic latent image on a photoconductive drum by irradiating a laser beam (a light beam) on the surface of a photoconductive drum and scanning the laser beam.
Recently, besides a monochrome machine including a scanning optical system that uses a single light source, a tandem color machine is proposed. For the tandem color machine, a method of increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) in one laser unit (a multi-beam method) is proposed for the purpose of realizing an increase in speed of scanning on the surface of a photoconductive drum. In the multi-beam method, plural beams for each of color components (e.g., yellow, magenta, cyan, and black) emitted from the respective light sources are subjected to processing in a pre-deflection optical system and are changed to one beam and made incident on a polygon mirror. The beam deflected by the polygon mirror is, after passing through an fθ lens configuring a post-deflection optical system, separated into beams for each of the color components and irradiated on a photoconductive drum for each of the color components.
In JP-A-7-256926, an optical beam scanning apparatus that can provide a color image without color drift at low cost and an image forming apparatus including the optical beam scanning apparatus are known. According to a technique proposed in JP-A-7-256926, the optical beam scanning apparatus includes a finite focal lens and a cylindrical lens that give a predetermined characteristic to lights from plural light sources, a mirror block that unites the lights passing through the lenses, a deflecting device that deflects the united light, first to third shared imaging lenses that adjust an aberration characteristic in a focus position of the deflected light, and one to three reflection mirrors that emit the light passing through the respective imaging lenses in predetermined positions. The reflection mirrors are movable. It is possible to adjust parallelism of a scanning line by moving the reflection mirrors.
Recently, there is also proposed an optical beam scanning apparatus in which an individual imaging lens is provided for each of color components in a post-deflection optical system in order to improve optical accuracy in a scanning optical system in the optical beam scanning apparatus. With the technique proposed in JP-A-7-256926, concerning an optical beam scanning apparatus in which all imaging lenses provided in a post-deflection optical system are shared lenses, it is possible to adjust parallelism of a scanning line by moving the reflection mirrors. However, concerning the optical beam scanning apparatus in which an individual imaging lens is provided for each of color components in the post-deflection optical system, it is difficult to adjust parallelism of a scanning line.